Reinforced chemical conversion coating

ABSTRACT

A process of preventing crack formation in chemical conversion coating comprising providing a substrate; introducing a chemical conversion coating material; adding an additive containing a binder to the conversion coating material; and coating the substrate with the chemical conversion coating and the additive.

BACKGROUND

The present disclosure is directed to the use of binders that preventcrack formation in chemical conversion coatings when those coatings cureand dry to assist with the protective film formation on metal surfaces.

The oxidation and degradation of metals used in aerospace, commercial,and private industries are a serious and costly problem. To prevent theoxidation and degradation of the metals used in these applications aninorganic protective coating can be applied to the metal surface. Thisinorganic protective coating, also referred to as a conversion coating,may be the only coating applied to the metal, or the coating can be anintermediate coating to which subsequent coatings are applied.

Chromate based coatings are currently used as inorganic conversioncoatings because they provide corrosion resistant properties andadhesion to subsequently applied coating layers. However, due toenvironmental and health and safety concerns over use of chromate basedcompounds, there is a need for an environmentally safer and lesshazardous replacement that provides equivalent corrosion inhibition andprotection to the underlying metal substrate surface, as well asadhesion to subsequently applied coatings.

Chemical Conversion Coating processes have been developed that usetrivalent chromium instead of hexavalent chromium species due to changesin environmental regulations and in order to prevent exposure of workersin the metal finishing industry to hexavalent chromium species.

Conversion coatings are typically applied at very low film thicknesses.Slight defects in these very thin films will expose the underlying metalto the corrosive environment and compromise the conversion coating'scorrosion inhibition and protection function, leading to substratecorrosion damage, such as pitting in the case of aluminum alloysubstrates.

SUMMARY

In accordance with the present disclosure, there is provided a processof preventing crack and pin-hole formation in chemical conversioncoating films comprising: providing a substrate; introducing a chemicalconversion coating material; adding a binder to the conversion coatingmaterial; and coating the substrate with the chemical conversion coatingand the additive.

In another and alternative embodiment, the binder is selected from thegroup consisting of polyvinyl-alcohol, polyvinylpyrrolidone,polyvinyl-acetate, styrene-butadiene and hydro-propyl-cellulose.

In another and alternative embodiment, the process further comprisesstrengthening the chemical conversion coating during formation byreducing differential stresses caused during drying and curing of thechemical conversion coating with the binder.

In another and alternative embodiment, the process further comprisesbinding transition metal ions in the chemical conversion coating.

In another and alternative embodiment, the process further comprisespreventing cracks in the chemical conversion coating.

In another and alternative embodiment, the chemical conversion coatingcomprises trivalent chromium.

In another and alternative embodiment, the process further comprises amixture of the binders.

In accordance with the present disclosure, there is provided a componentcomprising a substrate having a surface; and a chemical conversioncoating bonded to the surface, wherein the chemical conversion coatingcomprises a binder configured to prevent crack formation responsive tocuring.

In another and alternative embodiment, the chemical conversion coatingis a composite of trivalent chromium and binder.

In another and alternative embodiment, the binder is selected from thegroup consisting of polyvinyl-alcohol, polyvinylpyrrolidone,polyvinyl-acetate, styrene-butadiene and hydro-propyl-cellulose.

In another and alternative embodiment, the binder is configured to bindtransition metal ions in the chemical conversion coating.

In another and alternative embodiment, the component comprises a metalmaterial.

In another and alternative embodiment, a strength of the chemicalconversion coating having the binder is greater than a tensile stress ofthe chemical conversion coating generated during curing.

In another and alternative embodiment, the component further comprises amixture of the binders.

In another and alternative embodiment, the chemical conversion coatingcomprises trivalent chromium.

Other details of the use of binders that prevent crack formation inchemical conversion coatings are set forth in the following detaileddescription and the accompanying drawing wherein like reference numeralsdepict like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a conversion coating applied toa substrate;

FIG. 2 is a schematic representation of the conversion coating of FIG. 1after a period of curing;

FIG. 3 is a schematic representation of a conversion coating having anadditive applied to a substrate;

FIG. 4 is a schematic representation of the conversion coating withadditive of FIG. 3 after a period of curing.

DETAILED DESCRIPTION

Referring now to FIG. 1, there is illustrated a component 10. Thecomponent 10 includes a metal substrate 12, for example aluminum alloy,with a surface 14.

Applied to the surface 14 of substrate 12 is a chemical conversioncoating film 16. The chemical conversion coating 16 can be very thinwith for instance a thickness of about 200 nm. The chemical conversioncoating 16 can be viewed as a thin membrane of the chemical conversioncoating material that forms during the immersion process or duringtouch-up and repair activities.

The chemical conversion coating 16 can be applied as a solution ordispersion. The thin film after application to surface 14 can be driedand cured under controlled or uncontrolled humidity conditions.

Referring to FIG. 2, the thin film of the chemical conversion coating 16applied on the surface 14 of substrate 12 has complex mechanicalproperties that change over time during drying and curing. Evaporationof water from the as applied film 16 during drying, as well as coatingcuring leaves voids and makes the film shrink both in the perpendicularand lateral to the substrate surface 14 directions 18 and 20. The abovelead to the development of pin-holes and cracks 22 in the dried andcured coating film 16, that leave the substrate surface 14 directlyexposed to the corrosive environment and make it susceptible tocorrosion damage.

Referring now to FIGS. 3 and 4, a component 110 includes a substrate 112having a surface 114. A chemical conversion coating film 116 comprisingan additive 118 is applied to the surface 114. The additive 118 includesa binder material system 120. The additive 118 can be added to anyvariety of solutions or dispersions that are used as chemical conversionthin films 116 used for corrosion protection of the surface 114.

The binder materials 120 reduce the stresses in the film 116 when thesolution of the chemical conversion coatings 116 dry and cure. Thebinders 120 strengthen the gel-like network of the corrosion protectivecoatings 116 during formation and reduce any differential stresses thatoriginate from drying of thin films with non-uniform structures. Thebinders 120 can polymerize when exposed to chemical agents or radiationand can create a strong bond within the film 116 and between the film116 and the underlying metal alloy substrate 112.

After being deposited, as the film 116 dries, it shrinks in alldirections 124, both laterally and perpendicularly to the metal surface114. However, due to the binder 120, the film 116 does not form cracks.The film coating 116 remains completely covered over the surface 114.

In an exemplary embodiment, the additive 118 can comprise a binder 120comprising poly-vinyl alcohol (PVA) having a molecular weight of about72,000 g/mol can be dissolved in water at a concentration of about0.1-3.5 g/100 cubic centimeters (cm3). The solution can be added to themedia that is used to coat the surface of the metal substrate with thechemical conversion coating 116 corrosion protection so that the PVAconcentration will be about 0.02-1.4 g/100 cubic centimeters (cm3).

In another exemplary embodiment, the additive 118 can comprise a binder120 comprising hydro-propyl-cellulose (HPC) having a molecular weight of105 g/mol. HPC can be added to a solution with PVA binder in order tofurther control the drying stresses in the thin film chemical conversioncoating 116 used for protection to the underlying metal surface 114. TheHPC concentration in water is about 0.35 g/100 cm3, which can be dilutedfurther to about 0.12 g/100 cm3 by mixing with the solution thatcontains the chemical conversion coating 116.

In another exemplary embodiment, another type of binder 120 can bepolyvinylpyrrolidone (PVP), also at about 0.5 g PVP per 100 cm3. The PVPcan bind transition metal ions, such as, trivalent chromium, orzirconium ions. Other exemplary binders 120 can includestyrene-butadiene binder and polyvinyl-acetate.

A benefit of the additive 118 with the chemical conversion coating 116is that trivalent conversion coatings with a binder system 120 will haveless corrosion damage in a certain time frame. The trivalent conversioncoatings with the additive having a binder system 120 will therebyprovide a better protection of the metal substrate 112 againstcorrosion.

The binders 120 that have been discussed in the detailed description arepresent in a low concentration but play a critical role in making thethin films of the chemical conversion coating 116 more elastic and thebinders 120 thereby prevent the thin films from cracking during dryingand curing.

There has been provided the use of binders that prevent crack formationin chemical conversion coatings. While the use of binders that preventcrack formation in chemical conversion coatings has been described inthe context of specific embodiments thereof, other unforeseenalternatives, modifications, and variations may become apparent to thoseskilled in the art having read the foregoing description. Accordingly,it is intended to embrace those alternatives, modifications, andvariations which fall within the broad scope of the appended claims.

What is claimed is:
 1. A process of preventing crack and pin-holeformation in chemical conversion coating films comprising: providing asubstrate; introducing a chemical conversion coating material; adding abinder to said conversion coating material; and coating said substratewith said chemical conversion coating and said additive.
 2. The processaccording to claim 1, wherein said binder is selected from the groupconsisting of polyvinyl-alcohol, polyvinylpyrrolidone,polyvinyl-acetate, styrene-butadiene and hydro-propyl-cellulose.
 3. Theprocess according to claim 1, further comprising strengthening saidchemical conversion coating during formation by reducing differentialstresses caused during drying and curing of said chemical conversioncoating with said binder.
 4. The process according to claim 1, furthercomprising: binding transition metal ions in said chemical conversioncoating.
 5. The process according to claim 1, further comprising:preventing cracks in said chemical conversion coating.
 6. The processaccording to claim 1, wherein said chemical conversion coating comprisestrivalent chromium.
 7. The process according to claim 1, furthercomprising: a mixture of said binders.
 8. A component comprising: asubstrate having a surface; and a chemical conversion coating bonded tosaid surface, wherein said chemical conversion coating comprises abinder configured to prevent crack formation responsive to curing. 9.The component according to claim 8, wherein said chemical conversioncoating is a composite of trivalent chromium and binder.
 10. Thecomponent according to claim 9, wherein said binder is selected from thegroup consisting of polyvinyl-alcohol, polyvinylpyrrolidone,polyvinyl-acetate, styrene-butadiene and hydro-propyl-cellulose.
 11. Thecomponent according to claim 9, wherein said binder is configured tobind transition metal ions in said chemical conversion coating.
 12. Thecomponent according to claim 8, wherein said component comprises a metalmaterial.
 13. The component according to claim 8, wherein a strength ofsaid chemical conversion coating having said binder is greater than atensile stress of said chemical conversion coating generated duringcuring.
 14. The component according to claim 8 further comprising: amixture of said binders.
 15. The component according to claim 8, whereinsaid chemical conversion coating comprises trivalent chromium.